This invention relates to the refining of crude fatty oils.
The oil to be refined may be naturally free of phospholipids, as are palm oil, coconut oil and the other lauric acid oils, and animal fats. Or it may be a crude oil which contains phospholipids, as do soybean oil and safflower seed oil, for example. In the latter instance, the process includes degumming as a first step. How degumming is achieved is not part of this invention; usually the phospholipids are removed by a well-known technique of water hydration and centrifugal separation.
In the United States such oils have heretofore been refined by a caustic refining process. This process resulted in high losses and usually required further refining processes.
The present invention employs steam distillation and does not utilize caustics.
Perhaps one reason for the historical popularity of the caustic refining process in the United States was that until the 1940's, cottonseed was the principal source of crude oil for shortening or salad oil in the United States. This dark crude oil could not be steam-refinined, due to color problems. It was therefore necessary to caustic-refine this oil for fatty-acid removal as well as for removal of color bodies.
Cottonseed oil was followed as the popular salad or shortening oil by soybean oil, which has also been difficult to steam-refine, because the principal non-oil constituents of this oil are phospholipids. It is now known that these components must be thoroughly removed from the oil prior to high-temperature steam deodorization, and I have found that such removal makes steam refining quite practical. Phospholipids vary in their composition, primarily due to growing conditions of the basic soybean. At one time caustic refining was thought to be the only practical way to remove phospholipids, and even then, when certain types, such as nonhydratable phospholipids, were present, those required a special acid pretreatment before the caustic refining, followed by intensive bleaching with activated clay in order to produce an acceptable product.
However, this standard caustic refining process, has been difficult to use for refining certain types and grades of oil, and the losses have been high in any event. For example, for palm oil published refining losses have usually ranged from a low of 1.7 times free-fatty-acid (FFA) to over 2.2 times FFA. On a crude oil of 4.0% FFA with an average refining factor of 2.0, the loss would thus be about 8%, a very significant loss, leaving a refined yield of only 92 pounds for every 100 pounds of crude oil purchased. Even then, the caustic-refined palm oil would still have to be bleached and steam-deodorized.
The large amount of soapstock produced from this 8% refining loss resulting from caustic refining resulted in a difficult by-product that had to be processed further. It has usually been acidulated with sulfuric acid to make a low-grade fatty-acid oil. The effluent from the acidulation process is very high in biochemical oxygen demand and has had to be neutralized and treated before discharge into a sewer system.
Yet, palm oil has a natural versatility among edible oils because of its glyceride structure. It has good non-foaming properties and oxidative stability that makes it highly suitable for use in commercial deep frying. The plantations of Malaysia are now producing high-quality palm oil in sufficient quantities to permit its use in the production of frying fats, as well as margarine and shortenings, and the present invention enables its refinement with only negligible losses and enables recovery of salable by-products.
Palm oil, if properly refined has a composition that makes it a nearly ideal food fat substance. One disadvantage, however, has been the presence of from 400-800 ppm of carotenoid pigments, particularly formed from .alpha. and .beta. carotenes. These products impart to palm oil a dark red color that is unacceptable for a food fat.
The normal method of "bleaching" palm oil has, in the past, consisted of holding the palm oil, for one hour or more, under vacuum, at temperatures on the order of 240.degree. C. Under these conditions, the carotenes have undergone successive intramolecular Diels-Alder reactions; one double bond reacted with two conjugated double bonds to form a six-carbon ring, with diminished unsaturation. .beta.-carotene has 40 carbon atoms and 11 chromophoric conjugated double bonds. After bleaching by this prior-art heat treatment, the largest part has been recovered in the form of a mixture of polycyclic products which still contained 40 carbon atoms, but for which the number of double bonds was on the order of 2 to 3 per molecule. These products were colorless or very faintly colored; however, they were not eliminated either by treatment with decolorizing earth nor by deodorization. The volatile fraction particularly consisting of toluene, xylenes, and the methylnaphthalenes represented on the order of only 15 to 20% of the carotene.
When palm oil was, instead, "bleached" by treatment with decolorizing earths at temperatures between 100.degree. and 180.degree. C., the decolorization was due principally to the adsorption of pigments but more particularly to an isomerization catalyzed by the earth; furthermore, this bleaching, called "mixed", produced a high proportion of polymers of polyunsaturated fatty acids.
Both these classical procedures for decolorization of palm oil presented the added disadvantage of leaving, in the refined oil, substances extremely suspect from the toxicological point of view.
An important consideration in the pretreatment portion of the present invention is the removal of most of the .beta.-carotene prior to the high temperature treatment by steam refining. .beta.-carotene is a heat degradable material which is converted to a colorless polycyclic non-volatile product by conventional heat bleaching; however, the heat degraded residues which then remain can seriously reduce stability. By removal of the .beta.-carotene prior to high temperature treatment, a more stable finished edible oil is produced. Typical good crude palm oil may contain 450-500 ppm .beta.-carotene. The filtered oil, after the metal removal and clay bleaching steps according to the present invention, contain only 25-30 ppm .beta.-carotene, more than 90% of the carotene being removed by the metal removal and bleaching steps. This treated oil is then used as the feed stock which is steam refined and deodorized to produce the finished edible oil.
Deacidification of certain high-FFA crude oils using steam distillation as a primary refining step has been carried out in Europe for many years as a replacement for the basic caustic refining method. In practice of this method; it had been found difficult to reduce the free-fatty-acid content of the oils below 0.1-0.2%. As such, the steam distillation was usually carried out to reduce the FFA to the 0.5-0.6% range, and then the refining was completed by the basic caustic refining process. In order to produce a finished edible oil, the caustic refining step still had to be followed by conventional bleaching and steam deodorization.
High quality finished edible oil can only be produced from good quality crude oils. Crude palm oils contain natural antioxidants, for example tocopherols, which help in reducing the rate of oxidation while in the crude oil. Crude palm oil can be further protected at the time of shipment, e.g., from Malaysia, by the addition of further antioxidants, as well as protection by nitrogen blanketing, thereby helping to reduce oxidation during transit. A sample of crude palm oil which had 0.01% TBHQ (tert. butyl hydroquinone) added before shipment was pretreated, steam refined, and deodorized according to the present invention to produce a finished oil color of less than 0.6R.
While the quality of crude palm oil as obtained at the plantations, for example Malaysia, has improved greatly over the years due to a better understanding of the technology in processing the oil from the palm pod, oxidation during the refining processes has caused trouble. It is important to minimize oxidation in and of the crude oils.
Technical studies have shown a definite relationship between the total oxidation values of a fat and the keeping quality of the finished product. The total oxidation values, or "Totox," is an empirical value derived from the peroxide value [PV] and the anisidine value [AV]. Therefore Totox = [2 .times. PV] + AV where PV is a measure of the peroxides formed and the AV is a measure of the secondary oxidation products.
In a series of palm oils which were processed by conventional caustic refining, bleaching and deodorization, the average AV of the deodorized oil was 6.0 which, with a zero PV, gave a Totox of 6.0. In a review of quality requirements for palm oil used in margarine, it has been reported that a good margarine oil should have a Totox below 4.0. Previously this value has been unattainable.
Typical good crude palm oils show the following analyses:
Free fatty acid [FFA] = 3.2%-4.2% [average 3.5%]
.beta.-carotene = 450-500 ppm
Peroxide value [PV] = 3-6 milliequivalents/Kg
Anisidine value [AV] = 4-7
TOTOX = 10-19 [average 12-15]
Such crude palm oil, when processed from crude to deodorized finished oil by the process of the present invention results in finished deodorized edible oil with the following analyses:
Color = 1.5R - 2.5R [51/4 in. Lovibond]
FFA = 0.02%
.beta.-carotene = 35-70 ppm
PV = 0.0
AV = 1.0 - 3.0
TOTOX = 1.0 - 3.0
Further illustrating the relationship of crude oil quality, a parcel of crude palm recently received had an AV of 2.0 with a Totox of only 10.6. The finished deodorized oil produced by the process of the present invention analyzed with a color less than 10Y-1.0R and a Totox of 0.8.
It has been reported that caustic-soda refining removed or destroyed a part of the natural anti-oxidants, and hence the refined oil is not as stable as the crude oil. This became apparent when crude oils were caustic-soda refined at the plantations and shipped as a once-refined oil [only caustic-soda refined]. Certain of these oils have analyzed, upon receipt in the U.S.A., with Totox value between 60-100.
Oils, other than crude palm, have also been processed by the present process using this laboratory technique. Among those successfully processed have been soybean oil, corn oil, peanut oil, sesame oil, olive oil, coconut oil, babassu and palm kernel oils, tallows and lard.
Each type of crude oil usually requires some minor variation in the pretreatment prior to high temperature steam refining and deodorization. Oils containing phospholipids, for example soybean oil, require a thorough water degumming to remove all or nearly all water hydratable phosphorous compounds. The gums from the water degumming operation may be dried and sold as animal feed, or if the crude oil is free of meal, the dried material may be sold as edible lecithin. The finished deodorized soybean oil was equal to or better than deodorized oil produced by the conventional processing, in its keeping quality characteristics.
The principal advantage for steam refining and deodorizing a low acidity oil such as soybean, peanut et al, is in the reduction of plant pollution commonly caused by the acidulation of soapstock produced from conventional caustic refining.
Thus, one object of the invention is to provide an improved oil refining process that significantly reduces refining losses and at the same time enables use of a large proportion of the materials removed during the refining process.
Another object is to provide a simple continuous refining process requiring little labor and minimum supervision.
Another object is to provide a greatly improved steam refining process for crude fatty oils.
A further object is to enable the use of relatively small amounts of clay and other refining materials and to eliminate many refining steps so as to hold down the overall cost of refining.
Another aspect of the invention relates to pretreatment of the crude oil to remove trace metals, before the steam deodorization step.
A still further object is to produce economically a refined glyceride oil having a low color value and suitable for use in margarine or foods.
Trace metals, especially such as copper (Cu) and iron (Fe) are harmful to vegetable oils, especially in regard to their stability. It has now been found that these trace metals are especially harmful if they are present during the high-temperature treatment which is necessary for steam refining and deodorization. Heretofore, there was no appreciation of the extent of the trace metal problem and its effects on steam refining, nor were adequate measures practical for the removal of these trace metals.
Thus, other important objects of the invention are (1) to prevent the accumulation of trace metals in the oil before and during the refining process, and (2) to reduce greatly the trace metal content of the oil during the early steps of the refining process in order to improve the effectiveness of steam refining and deodorization. Achieving these objects makes it possible for the steam refining process to be adequate without incorporating any caustic refining process.